Method for the production of superabsorbers

Abstract

The present invention relates to a method for the continuous production of superabsorbers, wherein: superabsorber particles are coated by spraying-on a surface-post-crosslinking-agent solution; the coated superabsorber particles are thermally surface-post-crosslinked in a contact dryer (1); the thermally surface-post-crosslinked superabsorber particles are cooled in a contact dryer (2) having two horizontal shafts; and a solid particulate material, an aqueous solution and / or a dispersion are metered in each case by means of at least one first nozzle in the region of the first horizontal shaft and at least one second nozzle in the region of the second horizontal shaft in the contact dryer (2).

Description

[0001] Process for producing superabsorbents

[0002] The present invention relates to a process for the continuous production of superabsorbents, wherein superabsorbent particles are coated by spraying on a surface postcrosslinker solution, the coated superabsorbent particles are thermally surface postcrosslinked in a contact dryer 1, the thermally surface postcrosslinked superabsorbent particles are cooled in a contact dryer 2 with two horizontal shafts and a particulate solid, an aqueous solution and / or a dispersion is metered in the contact dryer 2 by means of at least one first nozzle in the region of the first horizontal shaft and at least one second nozzle in the region of the second horizontal shaft.

[0003] Superabsorbents are used in the manufacture of diapers, tampons, sanitary pads, and other hygiene products, as well as as water-retaining agents in agricultural horticulture. Superabsorbents are also known as water-absorbing polymers.

[0004] The production of superabsorbents is described in the monograph ''Modern Superabsorbent Polymer Technology”, FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103.

[0005] To improve application properties such as gel bed permeability (GBP) and absorption under a pressure of 49.2 g / cm 2 (AUL0.7 psi), superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, which increases absorption under a pressure of 49.2 g / cm 2(AUL 0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface crosslinking can be carried out in the aqueous gel phase. Preferably, however, dried, ground, and sieved polymer particles (base polymer) are coated on the surface with a surface crosslinker and thermally surface crosslinked. Suitable crosslinkers for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles.

[0006] The object of the present invention was to provide an improved process for coating surface-crosslinked superabsorbent particles, in particular uniform coating and avoiding agglomerates.

[0007] The object was achieved by a process for the continuous production of superabsorbents, wherein superabsorbent particles are coated by spraying on a surface postcrosslinker solution, the coated superabsorbent particles are thermally surface postcrosslinked in a contact dryer 1 and the thermally surface postcrosslinked superabsorbent particles are cooled in a contact dryer 2, characterized in that the contact dryer 2 has two horizontal shafts with mixing tools, the speed of the mixing tools corresponds to a Froude number of 0.005 to 0.25 and a particulate solid, an aqueous solution and / or a dispersion is metered in the contact dryer 2 by means of at least one first nozzle in the region of the first horizontal shaft and at least one second nozzle in the region of the second horizontal shaft.

[0008] For the purposes of this invention, the horizontal wave located to the right in the product flow direction is the first wave and the horizontal wave located to the left in the product flow direction is the second wave.

[0009] Contact dryers suitable for the continuous process according to the invention include paddle dryers and disc dryers. In contact dryers, the materials to be dried are guided along a heated surface by means of a dynamic tool and are then layered. Contact dryers can also be used for cooling.

[0010] The speed of the mixing tools corresponds to a Froude number of preferably 0.01 to 0.21, particularly preferably 0.02 to 0.18, most particularly preferably 0.05 to 0.15.

[0011] For mixers with horizontally mounted mixing tools, the Froude number is defined as follows: with r: radius of the mixing tool a> : angular frequency g: acceleration due to gravity

[0012] The particulate solid, the aqueous solution, and / or the dispersion can be metered into the contact dryer 2 below the product bed surface, preferably at least 10 mm, more preferably at least 50 mm, and most preferably at least 100 mm below the product bed surface. The particulate solid can be metered into the contact dryer 2 dispersed in a gas stream. The aqueous solution and / or the dispersion can be metered into the contact dryer 2 using a two-fluid nozzle or a hydraulic nozzle.

[0013] Dual-fluid nozzles enable atomization into fine droplets or a spray mist. The atomization shape is a circular or elliptical solid or hollow cone. Dual-fluid nozzles can be designed with external or internal mixing. With externally mixing dual-fluid nozzles, the liquid and atomizing gas leave the nozzle head through separate openings. They are only mixed in the spray jet after exiting the spray nozzle. This enables independent control of droplet size distribution and throughput over a wide range. The spray cone of the spray nozzle can be adjusted via the air cap position. With internally mixing dual-fluid nozzles, the liquid and atomizing gas are mixed within the spray nozzle, and the two-phase mixture leaves the nozzle head through the same bore or through several bores connected in parallel.With the internally mixing two-fluid nozzle, the flow and pressure ratios are more closely linked than with the externally mixing spray nozzle. Therefore, slight changes in flow rate lead to changes in the droplet size distribution. Adjustment to the desired flow rate is achieved via the selected cross-section of the nozzle bore.

[0014] Compressed air, gas, or steam at 0.5 bar and above can be used as the atomizing gas. The droplet size can be individually adjusted by adjusting the ratio of liquid to atomizing gas as well as the gas and liquid pressure.

[0015] The particulate solid, the aqueous solution, and / or a dispersion are each metered below the product bed surface in contact dryer 2 using at least two opposing nozzles. The opposite sides are determined by the position of the two horizontal shafts and the conveying direction. The particulate solid, the aqueous solution, and / or the dispersion must be metered via at least one nozzle on the right side of contact dryer 2 and at least one nozzle on the left side of contact dryer 2. Contact dryer 2 can also have several such nozzle pairs. It is advantageous if the two nozzles of a nozzle pair are approximately opposite each other.

[0016] In contact dryer 2, the angle between the horizontal shaft and the nozzle is preferably approximately 90°. The particulate solid, the aqueous solution, and / or the dispersion can be fed vertically from above. Feeding from below or obliquely from the side is also possible, with the angle to the vertical preferably being between 0 and 90°, more preferably between 0 and 70°, and most preferably between 0 and 50°. The oblique arrangement of the feed allows the use of shorter feeds and thus lower mechanical stresses during operation of contact dryer 2.

[0017] The present invention is based on the finding that particulate solids, aqueous solutions, and dispersions are difficult to mix evenly in the cooler (contact dryer 2). This is due to the insufficient mixing performance of the contact dryer. Therefore, it is necessary to meter the particulate solid, the aqueous solution, or the dispersion on both sides of the contact dryer 2.

[0018] The temperature of the superabsorbent particles when spraying on the surface postcrosslinker solution is preferably from 30 to 80°C, particularly preferably from 35 to 75°C, most preferably from 40 to 70°C.

[0019] The surface postcrosslinker solution preferably contains from 0.001 to 2 wt.%, particularly preferably from 0.01 to 1 wt.%, very particularly preferably from 0.03 to 0.7 wt.%, of a surface postcrosslinker, based in each case on the superabsorbent particles. The surface postcrosslinker solution further preferably contains from 0.5 to 5 wt.%, particularly preferably from 1.0 to 4 wt.%, very particularly preferably from 1.5 to 3 wt.%, of water, based in each case on the superabsorbent particles.

[0020] The superabsorbent particles are heated in the contact dryer 1 to a temperature of preferably 110 to 220°C, more preferably 120 to 210°C, most preferably 130 to 200°C. The residence time of the superabsorbent particles in the contact dryer 1 is preferably 10 to 60 minutes, more preferably 15 to 50 minutes, most preferably 20 to 40 minutes.

[0021] The contact dryer 1 and the connection to the contact dryer 2 can be trace heated and / or thermally insulated.

[0022] The amount of particulate solid used is preferably from 0.001 to 2.0 wt. %, more preferably from 0.01 to 1.0 wt. %, most preferably from 0.1 to 0.5 wt. %, based in each case on the superabsorbent particles. The average particle size of the particulate solid is preferably from 0.1 to 100 μm, more preferably from 0.5 to 50 μm, most preferably from 1 to 25 μm. The average particle size is the volume-average particle size and can be determined by light scattering. A suitable particulate solid is aluminum trihydroxide, silicon dioxide and / or aluminum oxide. The temperature of the superabsorbent particles during coating with the particulate solid is preferably less than 180°C, more preferably less than 160°C, most preferably less than 140°C.

[0023] The superabsorbent particles are cooled in the contact dryer 2 to a temperature of preferably 30 to 80°C, more preferably 35 to 70°C, most preferably 40 to 60°C. The residence time of the superabsorbent particles in the contact dryer 2 is preferably from 10 to 60 minutes, more preferably from 15 to 50 minutes, most preferably from 20 to 40 minutes.

[0024] The mixing tools of the contact dryer 2 have a diameter of preferably 0.2 to 2 m, particularly preferably 0.4 to 1.2 m, very particularly preferably 0.6 to 1.2 m. The rotational speed of the mixing tools is preferably less than 25, particularly preferably less than 20, very particularly preferably less than 15, revolutions per minute.

[0025] The production of superabsorbents is explained in more detail below:

[0026] The superabsorbents are produced by polymerizing a monomer solution and are usually water-insoluble.

[0027] The ethylenically unsaturated, acid group-bearing monomers are preferably water-soluble, ie the solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, particularly preferably at least 25 g / 100 g water, most particularly preferably at least 35 g / 100 g water.

[0028] Suitable monomers include, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is most preferred.

[0029] The ethylenically unsaturated, acid-containing monomers are usually partially neutralized. Neutralization is carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably from 40 to 85 mol%, more preferably from 50 to 80 mol%, most preferably from 60 to 75 mol%, and the usual neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates, and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Sodium and potassium are particularly preferred as alkali metals, but very particular preference is given to sodium hydroxide, sodium carbonate, or sodium bicarbonate, and mixtures thereof, in particular sodium hydroxide.

[0030] The monomers usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers.

[0031] Suitable crosslinkers are compounds with at least two groups suitable for crosslinking. Examples of such groups include ethylenically unsaturated groups that can be radically polymerized into the polymer chain and functional groups that can form covalent bonds with the acid groups of the monomer. Polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinkers.

[0032] Suitable crosslinkers are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- and triacrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, WO 03 / 104301 A1 and DE 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated groups, as in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures as described for example in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0033] The amount of crosslinker is preferably 0.05 to 1.5 wt.%, particularly preferably 0.1 to 1 wt.%, most preferably 0.15 to 0.6 wt.%, in each case calculated on the total amount of monomer used. With increasing crosslinker content, the centrifuge retention capacity (CRC) and the absorption under a pressure of 21.0 g / cm decrease. 2 (AUL0.3psi) passes through a maximum.

[0034] Any compound that generates radicals under the polymerization conditions can be used as initiators, for example, thermal initiators, redox initiators, and photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. The disodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, and sodium bisulfite is preferably used as the reducing component. Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0035] The water content of the monomer solution is preferably from 40 to 75 wt.%, particularly preferably from 45 to 70 wt.%, and most preferably from 50 to 65 wt.%. As the water content increases, the energy required for subsequent drying increases, and as the water content decreases, the heat of polymerization can only be dissipated insufficiently.

[0036] The temperature of the monomer solution is preferably from 10 to 90°C, more preferably from 20 to 70°C, most preferably from 30 to 50°C.

[0037] The preferred polymerization inhibitors require dissolved oxygen for optimal effectiveness. Therefore, the monomer solution can be freed of dissolved oxygen before polymerization by inerting, i.e., by flowing an inert gas, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution before polymerization is preferably reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

[0038] Suitable reactors for polymerization include kneader reactors or belt reactors. In the kneader, the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating agitator shafts, as described in WO 2001 / 038402 A1. Belt polymerization is described, for example, in DE 38 25 366 A1 and US Pat. No. 6,241,928. Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example, in an extruder or kneader.

[0039] To improve the drying properties, the crushed polymer gel obtained by means of a kneader can be additionally extruded.

[0040] The polymer gel is then typically dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10 wt.%, particularly preferably 1 to 7 wt.%, most preferably 2 to 5 wt.%, wherein the residual moisture content is determined according to the EDANA-recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating." If the residual moisture content is too high, the dried polymer gel will have a glass transition temperature T too low. gand is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle, and undesirably large amounts of superabsorbent particles with too small a particle size ("fines") are produced in the subsequent comminution steps. The solids content of the polymer gel before drying is preferably between 25 and 90 wt. %, more preferably between 35 and 70 wt. %, and most preferably between 40 and 60 wt. %. The dried polymer gel is then crushed and optionally coarsely crushed.

[0041] The dried polymer gel is then usually ground and classified, whereby single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills can be used for grinding.

[0042] The average particle size of the superabsorbent particles separated as the product fraction is preferably from 150 to 850 pm, more preferably from 250 to 600 pm, and most preferably from 300 to 500 pm. The average particle size of the product fraction can be determined using the E-DANA recommended test method No. WSP 220.2 (05) "Particle Size Distribution," in which the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically. The average particle size is the mesh size value resulting for a cumulative 50 wt.%.

[0043] The superabsorbent particles are thermally surface-crosslinked to further improve their properties. Suitable surface-crosslinkers are compounds containing groups that can form covalent bonds with at least two carboxylate groups of the superabsorbent particles. Suitable compounds include, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1, and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 3523 617 A1, and EP 0 450 922 A2, or ß-hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230.

[0044] In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers.

[0045] The polyvalent cations usable in the process according to the invention include, for example, divalent cations, such as the cations of zinc, magnesium, calcium, and strontium; trivalent cations, such as the cations of aluminum, iron, chromium, rare earths, and manganese; and tetravalent cations, such as the cations of titanium and zirconium. Possible counterions include chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate, and carboxylates, such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0046] The amount of polyvalent cation used is, for example, 0.001 to 1.5 wt.%, preferably 0.005 to 1 wt.%, particularly preferably 0.02 to 0.8 wt.%, in each case based on the polymer.

[0047] Surface post-crosslinking is performed by spraying a solution of the surface post-crosslinker onto the dried superabsorbent particles. Following spraying, the superabsorbent particles coated with the surface post-crosslinker are thermally surface-crosslinked.

[0048] The spraying of a solution of the surface post-crosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disc mixers and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are most preferred. The distinction between horizontal mixers and vertical mixers is made by the bearing of the mixing shaft, i.e. horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, Horizontale Pflugschar® mixers (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands). However, it is also possible to spray the surface post-crosslinker solution in a fluidized bed.

[0049] Surface postcrosslinkers are typically used as an aqueous solution. The penetration depth of the surface postcrosslinker into the superabsorbent particles can be adjusted by varying the non-aqueous solvent content or the total solvent quantity.

[0050] Thermal surface post-crosslinking is carried out in contact dryers, particularly preferably paddle dryers, and most preferably disc dryers. Suitable dryers include, for example, Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA), and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). The surface-post-crosslinked superabsorbent particles can then be reclassified, with overly small and / or overly large superabsorbent particles being separated and returned to the process.

[0051] The surface-crosslinked superabsorbent particles can be coated or remoistened to further improve their properties.

[0052] Remoistening is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, and most preferably at 40 to 60°C. At temperatures that are too low, the superabsorbent particles tend to clump together, and at higher temperatures, water evaporates noticeably. The amount of water used for remoistening is preferably from 1 to 10 wt.%, particularly preferably from 2 to 8 wt.%, and most preferably from 3 to 5 wt.%. Remoistening increases the mechanical stability of the superabsorbent particles and reduces their tendency to static charge. Remoistening is advantageously carried out in the cooler after thermal surface postcrosslinking.

[0053] Suitable coatings for improving the swelling rate and gel bed permeability (GBP) include inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or multivalent metal cations. Suitable coatings for dust binding include polyols. Suitable coatings for counteracting the undesirable caking tendency of superabsorbent particles include fumed silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20.

[0054] Example 1 (according to the invention)

[0055] By continuously mixing deionized water, 48 wt.% sodium hydroxide solution, and acrylic acid, a monomer solution was prepared so that the degree of neutralization corresponded to 72.0 mol%. The water content of the monomer solution was 57.0 wt.%.

[0056] Triple-ethoxylated glycerol triacrylate (approximately 85 wt.%) was used as the crosslinker. The amount used was 0.99 kg per t of monomer solution. Polyethylene glycol with an average molecular weight of 4,000 g / mol was also added to the monomer solution. The amount used was 2.64 kg per t of monomer solution. To initiate the radical polymerization, 0.99 kg of a 0.25 wt.% aqueous hydrogen peroxide solution, 4.38 kg of a 15 wt.% aqueous sodium peroxodisulfate solution, and 0.88 kg of a 1 wt.% aqueous ascorbic acid solution were used per t of monomer solution.

[0057] The monomer solution was transferred into a List Contikneter reactor with a volume of 6.3m 3 (LIST AG, Arisdorf, Switzerland). The throughput of the monomer solution was approximately 20 t / h. The reaction solution had a temperature of 23.5°C at the inlet.

[0058] Between the addition point for the crosslinker and the addition points for the hydrogen peroxide and sodium peroxodisulfate solutions, the monomer solution was inerted with nitrogen. Ascorbic acid was metered directly into the reactor. Additionally, a 20 wt. % aqueous solution of the disodium salt of 1-hydroxyethylidene-1,1'-diphosphonic acid (etidronic acid) was metered into the reactor. The amount of solution used was 6.17 kg per t of monomer solution.

[0059] After approximately 50% of the residence time, an additional 1,000 kg / h of superabsorbent particles with a particle size of less than 150 pm, which had been generated during the crushing and classification process during the production process, were added to the reactor. The residence time of the reaction mixture in the reactor was approximately 15 minutes.

[0060] The resulting polymer gel was fed onto the conveyor belt of a circulating air belt dryer using an oscillating conveyor belt. The circulating air belt dryer was 48 m long and its conveyor belt had an effective width of 4.4 m. On the circulating air belt dryer, the aqueous polymer gel was continuously circulated with an air / gas mixture (approx. 175°C) and dried. The residence time in the circulating air belt dryer was 37 minutes.

[0061] The dried polymer gel was ground using a three-stage roller mill and sieved to a particle size of 150 to 710 pm. Superabsorbent particles with a particle size of less than 150 pm were separated. Superabsorbent particles with a particle size greater than 710 pm were returned to the grinding process. Superabsorbent particles with a particle size in the range of 150 to 710 pm were thermally surface-crosslinked.

[0062] The superabsorbent particles were coated with a surface post-crosslinker solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then thermally surface post-crosslinked in a NARA Paddle Dryer (Contact Dryer 1, GMF Gouda, Waddinxveen, Netherlands) for 45 minutes at 120°C. The following quantities were dosed into the Schugi Flexomix®:

[0063] 7.5 t / h superabsorbent particles

[0064] 361.5 kg / h surface post-crosslinker solution

[0065] The surface postcrosslinker solution contained 0.87 wt% ethylene glycol diglycidyl ether, 32.78 wt% 1,2-propanediol, 1.10 wt% aluminum sulfate and 65.25 wt% water.

[0066] The surface-crosslinked superabsorbent particles were transferred to a NARA paddle cooler (Contact Dryer 2, GMF Gouda, Waddinxveen, Netherlands) using a rotary valve and cooled to approximately 60°C. The surface-crosslinked superabsorbent particles were coated with 576.75 kg / h of an aqueous solution. The aqueous solution contained 0.65 wt.% aluminum sulfate and 0.065 wt.% sorbitan monolaurate (Span®20). The aqueous solution was metered below the product bed surface using two opposing dual-fluid nozzles. The angle to the vertical was 48°. The distance of the dual-fluid nozzles from the end wall was 1135 mm and 1280 mm. The blades had a diameter of approximately 0.9 m and rotated at approximately 10 revolutions per minute. The residence time was approximately 20 minutes.

[0067] The obtained superabsorbent particles contained very few agglomerates

[0068] Example 2 (not according to the invention)

[0069] The procedure is as in Example 1. The aqueous solution was dosed only through the first two-fluid nozzle. The distance of the two-fluid nozzle from the front wall was 1280 mm.

[0070] The obtained superabsorbent particles contained a large number of agglomerates

[0071] Example 3 (not according to the invention)

[0072] The procedure is as in Example 1. The aqueous solution was dosed using two consecutive dual-fluid nozzles. The distance between the dual-fluid nozzles and the front wall was 1280 mm and 2295 mm.

[0073] The obtained superabsorbent particles contained agglomerates.

[0074] REPLACEMENT SHEET (RULE 26)

Claims

Patent claims 1. A process for the continuous production of superabsorbents, wherein superabsorbent particles are coated by spraying on a surface postcrosslinker solution, the coated superabsorbent particles are thermally surface postcrosslinked in a contact dryer 1 and the thermally surface postcrosslinked superabsorbent particles are cooled in a contact dryer 2, characterized in that the contact dryer 2 has two horizontal shafts with mixing tools, the speed of the mixing tools corresponds to a Froude number of 0.005 to 0.25 and a particulate solid, an aqueous solution and / or a dispersion is metered in the contact dryer 2 by means of at least one first nozzle in the region of the first horizontal shaft and at least one second nozzle in the region of the second horizontal shaft.

2. Process according to claim 1, characterized in that the rotational speed of the mixing tools corresponds to a Froude number of 0.05 to 0.

15.

3. Process according to claim 1 or 2, characterized in that the particulate solid, the aqueous solution and / or the dispersion is metered in the contact dryer 2 below the product bed surface.

4. Process according to one of claims 1 to 3, characterized in that the residence time of the superabsorbent particles in the contact dryer 2 is from 10 to 60 minutes.

5. Process according to one of claims 1 to 4, characterized in that the amount of particulate solid, aqueous solution and / or dispersion used is in each case from 0.001 to 2.0 wt.%, based on the superabsorbent particles.

6. Method according to one of claims 1 to 5, characterized in that the mixing tools have a diameter of 0.2 to 2 m.

7. Method according to one of claims 1 to 6, characterized in that the rotational speed of the mixing tools is less than 25 revolutions per minute.

8. Process according to one of claims 1 to 7, characterized in that silicon dioxide, aluminum oxide and / or aluminum trihydroxide is used as the particulate solid.

9. The method according to any one of claims 1 to 8, characterized in that an aqueous solution of a polyethylene glycol, an aluminum salt and / or a surfactant is used as the aqueous solution.

10. Process according to one of claims 1 to 9, characterized in that the superabsorbent particles are cooled in the contact dryer 2 to a temperature of 30 to 80°C.

11. Process according to one of claims 1 to 10, characterized in that partially neutralized, crosslinked polyacrylic acid is used as superabsorbent.

12. Process according to one of claims 1 to 11, characterized in that the surface post-crosslinker can form covalent bonds with the superabsorbent.

13. Process according to one of claims 1 to 12, characterized in that the temperature of the superabsorbent particles when spraying the surface post-crosslinker solution is from 30 to 80°C.

14. Process according to one of claims 1 to 13, characterized in that the superabsorbent particles are heated in the contact dryer 1 to a temperature of 110 to 220°C.

15. Process according to one of claims 1 to 14, characterized in that the residence time of the superabsorbent particles in the contact dryer 1 is from 10 to 60 minutes.

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